When you revisit a slide in a presentation, PowerPoint only shows the slide as it was finally revealed at the end of any animations, sounds, or movies on the slide.
There's no direct way to re-start the animations and view the slide from the start, but if you insert a blank slide ahead of the slide with the animations, set it to automatically transition to the following slide after 0 seconds delay, you can jump back to this slide, it will automatically play forward to the following slide (the one with animations) and you're in business.